1. Field of the Invention
The present invention relates generally to image forming apparatuses such as printers and copiers capable of forming color images, and more particularly to an image forming apparatus that performs color registration control on an endless moving member at the time of forming color images.
2. Description of the Related Art
As such image forming apparatuses as mentioned above, those are known that form a color image by successively transferring and superposing color images onto a transfer belt (also referred to as “intermediate transfer belt”), which is a single endless moving member, or a single recording medium (such as a sheet of paper) conveyed by a conveyor belt, where the color images are formed on the image carriers of corresponding image forming parts arranged side by side along the rotational direction of the conveyor belt.
Techniques related to color misregistration correction in such image forming apparatuses are described in, for example, Patent Documents 1 through 4 listed below. For example, Patent Document 1 describes reading a series of misregistration detection pattern images formed on a transfer belt (conveyor belt) with a pattern detection sensor using optical sensors formed of a specular reflection optical system, calculating the amount of misregistration between each adjacent two of color images based on the read pattern images, and correcting the positions of the images formed by corresponding image forming parts of colors other than a reference color based on the calculated amounts of misregistration.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2001-312116    [Patent Document 2] Japanese Laid-Open Patent Application No. 2007-102189    [Patent Document 3] Japanese Laid-Open Patent Application No. 2006-235560    [Patent Document 4] Japanese Laid-Open Patent Application No. 2006-091141
However, according to the image forming apparatus described in Patent Document 1, the spot positions of specular reflection and diffuse reflection are offset relative to each other for various reasons in the pattern detection sensors, so that it is difficult to calculate an accurate center position from a pattern detection signal because of the effect of diffuse reflection light that enters the optical sensors together with specular reflection light. The resultant detection error has more than a small effect on the accuracy of color registration, thus preventing improvement in the accuracy of color registration.
Patent Document 2 describes a method of determining abnormality in the amount of correction based on a detected amount of color misregistration or setting fixed values at the time of adjustments in a factory, and is not related to control of reading color registration patterns.
The technique described in Patent Document 3 is for preventing a decrease in the accuracy of color misregistration detection due to variations in speed caused by variations in the thickness of a belt, and does not prevent a decrease in the accuracy of color misregistration detection due to sensor reading error.
The technique described in Patent Document 4 is related to correcting an error in color misregistration calculation in an easy and simplified manner in accordance with an environment of usage such as a temperature or humidity at the time, and is not related to control of reading color registration patterns.